Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases, and functions as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity.
Referring to FIG. 1, a partial section of a typical fuel cell 10 is detailed. In fuel cell 10, hydrogen gas 12 and reactant water 14 are introduced to a hydrogen electrode (anode) 16, while oxygen gas 18 is introduced to an oxygen electrode (cathode) 20. The hydrogen gas 12 for fuel cell operation can originate from a pure hydrogen source, methanol or other hydrogen source. Hydrogen gas electrochemically reacts at anode 16 to produce hydrogen ions (protons) and electrons, wherein the electrons flow from anode 16 through an electrically connected external load 21, and the protons migrate through a membrane 22 to cathode 20. At cathode 20, the protons and electrons react with the oxygen gas to form resultant water 14′, which additionally includes any reactant water 14 dragged through membrane 22 to cathode 20. The electrical potential across anode 16 and cathode 20 can be exploited to power an external load.
The same configuration as is depicted in FIG. 1 for a fuel cell is conventionally employed for electrolysis cells. In a typical anode feed water electrolysis cell (not shown), process water is fed into a cell on the side of the oxygen electrode (in an electrolysis cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolysis cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. In a cathode feed electrolysis cell (not shown), process water is fed on the hydrogen electrode, and a portion of the water migrates from the cathode across the membrane to the anode where protons and oxygen gas are formed. A portion of the process water exits the cell at the cathode side without passing through the membrane. The protons migrate across the membrane to the cathode where hydrogen gas is formed.
In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems.
The typical electrochemical cell includes a number of individual cells arranged in a stack, with the working fluid directed through the cells via input and out put conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. The anode, cathode, or both are conventionally gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane electrode assembly”, or “MEA”) is typically supported on both sides by support members such as screen packs or bipolar plates, forming flow fields. Since a differential pressure often exists in the cells, compression pads or other compression means are often employed to maintain uniform compression in the cell active area, i.e., the electrodes, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.
In addition to providing mechanical support for the MEA, flow fields such as screen packs and bipolar plates preferably facilitate fluid movement and membrane hydration. These porous support members can also serve as gas diffusion media to effectuate proper transport of the oxygen and hydrogen gas. Increasing the rates of transport and uniformity of distribution of the fluids (including liquid water and oxygen and hydrogen gases) throughout the electrochemical cells increases operating efficiencies.
With the support of the screen packs, conventional electrochemical cells can operate at a pressure differential of up to about 400 pounds per square inch (psi). While suitable for their intended purposes, such support members require additional manufacturing materials and steps, and may not be effective for cells operating at differential pressures greater than about 400 psi. In an electrolysis cell, for example, it is desirable to operate the cell at about 1,000 psi or greater. At pressures exceeding about 400 psi, and especially exceeding 600 psi, physical limitations of screen structures, i.e., the requirement of very small screen openings, hinders fluid transport therethrough, and thus limits their usefulness.
In order to enable operation at pressures up to about 2,000 psi, porous plate technology has disclosed in U.S. Pat. Nos. 5,296,109 and 5,372,689, issued to Carlson et at. in 1994. As shown in FIG. 2, a porous sheet 213 is disposed between the anode electrode 211 and the flow field (screen pack 203) to provide additional structural integrity to the membrane 209. According to Carlson, porous sheet 213 further enables dual-directional water and oxygen flow.
Porous plate have also been previously disclosed in a paper presented at The Case Western Symposium on “Membranes and Ionic and Electronic Conducting Polymer”, Cleveland, Ohio May 17-19, 1982. Again as illustrated in FIG 2, this paper discloses that in order to prevent the membrane and electrode assembly from deforming into the flow fields, a porous, rigid support sheet 213 is inserted between the electrode 211 and the flow field distribution component 203. The particular arrangement described employs a porous titanium support sheet on the anode electrode, and a carbon fiber paper, porous, rigid support sheet on the cathode electrode (p. 14). At page 2, this paper claims that such cells were capable of operating at differential pressures ranging up to greater than 500 psi.
Use porous plates are also disclosed in “Solid Polymer Electrolyte Water Electrolysis Technology Development for Large-Scale Hydrogen Production”, Final Report for the Period October 1977-November 1981 by General Electric Company, NTIS Order Number DE82010876, which is directed to solid polymer electrolyte water electrolysis technology. Certain electrolyzer arrangements using porous titanium plates are described, and as shown in FIG. 2 include an anode electrode 211, an anode electrode flow field of molded grooves 203, a cathode electrode 207, a cathode flow field of molded grooves 205, and ion exchange membrane 209 disposed between and in intimate contact with anode 211 and cathode 207. A porous sheet 213 is shown supporting ion exchange membrane 209 and interposed between anode flow field 203 and anode electrode 211. It is stated on page 10 of the report that earlier development had shown that a support for the membrane and electrode assembly was required on both the anode and cathode side to prevent creep of the membrane into flow areas. The anode support (porous sheet 213) comprised a thin, titanium foil with many small holes etched through for transport of water to the catalyst from the flow field and outflow of the oxygen gas. To provide improved water flow rates, these thin foil anode supports were reported to be replaced with porous titanium plates (p. 66).
A significant disadvantage of porous plate technology is the additional materials, manufacturing, and assembly expense that this element adds to the cell assembly. What is accordingly needed in the art is a cost effective electrochemical cell capable of operating at high pressures, e.g. exceeding about 1,000 psi.